Fix issue: enable to delete a gerrit server as site admin. Foreign keys in db were preventing deletion.

Change-Id: I8294120099f50397b58d4091a4524169c455ea3c
Merge commit 'refs/changes/75/1575/2' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  Fix issue: enable to delete a gerrit server as site admin. Foreign keys in db were preventing deletion.

Change-Id: I18755703a4cb2acc71ac54aca0f22183d3929236
This is Tuleap 6.8.99.5.

Change-Id: I3ae975977890136e7fe2238884c0c33a6a72e524